Physicists Made the Most Precise Measurement of Protons’ “Magic Moment”

In Brief

A team of scientists used precise methods to uncover the magnetic moment of protons. This is a property of particles that is a prerequisite for magnetism, and a fundamental aspect of atomic structure.

Protons’ Magnetic Moment

An international team of scientists employed highly precise methods to uncover the most exact measurement of the magnetic moment of protons. They found it to be 2.79284734462, plus-or-minus 0.00000000082 nuclear magnetons (the typical unit for measuring this property).

The magnetic moment is a property of particles that is a prerequisite for magnetism, and applied to protons, it embodies a fundamental property of atomic structure. The team included scientists from RIKEN’s Ulmer Fundamental Symmetries Laboratory (FSL), Johannes Gutenberg-Universität Mainz, Max Planck Institute for Nuclear Physics, Heidelberg, and GSI Darmstadt.

The level of precision for the first-of-a-kind measurements was better than one part per billion.

In order to achieve this kind of specificity, researchers needed to isolate a single proton. Not a microscopic handful or an iota of particles; just one, caught in a Penning trap. They detected the thermal signal of ions (atoms or molecules with an uneven ratio of electrons to protons) and used an electric field to eliminate protons until there was only one left.

A Penning trap. Image Credit: RIKEN

Achieving the high level of specificity for the experiment required both extremely difficult engineering and moving the proton between two types of traps.

Methodology and Purpose

A proton inside a Penning trap will sync its spin with the magnetic field inside the trap. The detector measured two frequencies: the the cyclotron frequency of the proton in a magnetic field and the Larmor (spin-procession) frequency. Together, these help determine the magnetic moment. After the proton goes through all that in the first trap, it moves to a second trap, where its spin state is obtained with a magnetic bottle.

Georg Schneider, first author of the study, says the work will “allow us to get a better understanding of, for example, atomic structure.” Andreas Mooser, member of RIKEN FSL and second author of the study, said, “Looking forward, using this technique, we will be able to make similarly precise measurements of the antiproton at the BASE experiment in CERN, and this will allow us to look for further hints for why there is no antimatter in the universe today.”